This invention relates to an improved control device for an elevator driven by an a.c. motor.
According to a known device of this kind, an elevator car is driven by an induction motor to which a current is supplied from an alternating current source of variable voltage and frequency to effect speed control of the motor. This known device is shown in FIG. 1.
In FIG. 1, the numeral 1 designates a rectifier device connected to a three-phase a.c. source R, S, T. The numeral 2 denotes an inverter formed e.g. by thyristors connected to the d.c. side of the rectifier device 1 and designed to convert the direct current into the alternating current with variable voltage and frequency in the manner known per se. The numeral 3 designates a three-phase induction motor driven by the inverter 2. The numeral 4 designates a brake wheel coupled to the motor 3. The numeral 5 designates a brake shoe mounted opposite to the outer periphery of the brake wheel 4 for braking the brake wheel 4 under the force of a spring, not shown. The numeral 6 designates a brake coil adapted when energized to disengage the brake shoe 5 from the brake wheel 4 against the force of the spring. The numeral 7 designates a driving sheave of a winch driven by the motor 3. The numeral 8 designates a main cable wound about the sheave 7. The numeral 9 designates a car coupled to the cable 8, and the numeral 10 a counterweight. The numerals 11a to 11c designate contacts of a magnetic contactor for ascent which is inserted between the inverter 2 and the motor 3 and closed when the car 9 travels towards above. The numeral 11d designates a contact of the magnetic contactor connected to the brake coil 6 and operating in the same manner as the contacts 11a to 11c. The numerals 12a to 12c designate contacts of a magnetic contactor for descent which is inserted between the inverter 2 and the motor 3 and closed when the car 9 travels towards below. The numeral 12d designates a contact of the magnetic contactor for descent connected in parallel with the contact 11d and operating in the same manner as the contacts 12a to 12c. The numeral 13 designates a direct current source connected across contacts 11d, 12d and brake coil 6.
In operation, while the car 9 is at a standstill, brake shoe 5 is pressured to the brake wheel 4 under the force of the spring. Since the cage 9 travels towards above, when the contact 11d of the magnetic contactor for ascent is closed, the brake coil 6 is energized and the brake shoe 5 is disengaged from the brake wheel 4. Simultaneously, the contacts 11a to 11c are closed, so that the a.c. power of variable frequency supplied as output from the inverter 2 is supplied to the motor 3. In this manner, the motor 3 is started, and the car 9 travels towards above. The a.c. power is controlled in frequency by the inverter 2 for controlling in turn the r.p.m. of the motor 3 and hence the travel speed of the car 9. When approaching the floor of destination, the cage 9 starts to be showed down.
The contacts 11a to 11c are opened shortly before the car gets to the floor of destination. Thus, the source is dis-connected from the motor 3. Simultaneously, the contact l1d is opened to deenergize the brake coil 6, so that the brake shoe 5 is pressured to the brake wheel 4 under the force of the spring. In this manner, the car 9 is brought to a stop. The car 9 may travel towards below in the similar manner as mentioned above.
It is required of an elevator to be operated smoothly and with a higher operating efficiency since the time of start until halt thereof through high speed travel and slowdown. Hence, the alternating current of the extremely low frequency must be supplied to the motor 3 at the start and shortly before the car comes to a standstill. On the other hand, the braking characteristics of the contacts 11a to 11c and 12a to 12c are such that the breakable current capacity is lowered with the decrease in frequency. In this consequence, when the frequency is lowered, a larger magnetic contactor must be used, even when the current intensity remains the same. Moreover, in case of a trouble of the inverter 2, a large direct current may flow through the motor 2. After all, the motor 3 must be able to be disengaged positively from the source at any power source frequency for assuring utmost safety of elevator operation. Hence, the contact is required to have a larger breaking capacity. In addition, it is necessary to provide two sets of contacts 11a to 11c and 12a to 12c, which means additional costs.